Do all neurodegenerative diseases such as Parkinson's, Alzheimer's, etc. show up on an MRI scan?
Neurodegenerative diseases are diagnosed on the basis of clinical symptoms (memory impairment for Alzheimer Disease/ bradykinesia, tremor and rigor for Parkinson Disease) +/- neuropsychological tests.
The current role of neuroimaging in the diagnosis of neurogenerative diseases such as Parkinson Disease or Alzheimer Disease is mainly supportive, it can be used
- to confirm a high suspicion for the disease (due to typical clinical presentation for example)
- to exclude possible other diseases when the clinical diagnosis is unclear
None of the current dignostic criteria for AD or PD rely solely on neuroimaging studies.
There are some signs on MRI scan which may indicate the presence of AD or PD but patients with advanced stages of the disease may show limited signs of the diseases on MRI.
In AD, patients typically present hippocampal atropy (craked walnut sign, see image below) or medio temporal atrophy. However, as hippocampal atrophy is also present with age, some age-specific criteria are needed to differentiate between a age-related hippocampal atrophy and an AD-related hippocampal atrophy. Other signs may be present such as white matter anomalies. But these signs do not correlate necessarily with the severity of the disease.
In PD, the role of MRI is very limited and is mainly used to help when a differential diagnosis for the symptoms is present. It can also help diagnosing between an idiopathic PD or a vascular PD for example (if signs of cerebrovascular diseases are present on the MRI).
Recently, advanced techniques of neuroimaging (non exhaustive list: functional brain imaging with [18F] FDG-PET, functional MRI (fMRI), perfusion MRI, SPECT, amyloid PET tracers (F18-florbetapir, F18-flutemetamol, F18-florbetaben) in AD and MR spectroscopy (MRS), magnetization transfer imaging, diffusion-weighted MRI, diffusion tensor MRI, and high-resolution imaging (eg, MRI at 7 Tesla) for PD) have been suggested to help in the diagnosis of AD and PD. However, most of the studies rely on a small number of patients so further studies including a larger sample size are needed to confirm the encouraging results.
Wolk D et al. Clinical features and diagnosis of Alzheimer disease. UpToDate. October 2016. https://www.uptodate.com/contents/clinical-features-and-diagnosis-of-alzheimer-disease?source=machineLearning&search=alzheimer&selectedTitle=1~150§ionRank=3&anchor=H406199175#H406199175
Chou K et al. Diagnosis and differential diagnosis of Parkinson disease. UpToDate. October 2016. https://www.uptodate.com/contents/diagnosis-and-differential-diagnosis-of-parkinson-disease?source=see_link#H17
for the picture: Ho M. and Eisenberg R. Neuroradiology signs. Alzheimer Disease, p19.